Many of the present well logging approaches in formation evaluation are focused on the determination of oil saturation and therefore measurements in fluid-filled formations. For nuclear measurements, oil and fresh water can appear similar. In particular, the hydrogen index (HI) of water and a typical oil is almost the same. As the densities of oil and water are different by a few tenths of a g/cm3, oil-filled and water-filled formations of the same overall hydrogen index will show a slight difference in bulk density.
Natural gas (methane) has a lower density than oil but contains about twice the number of hydrogen atoms per carbon atom. In open hole (OH) logging, the detection and quantification of gas is often based on a neutron-density cross over. While the neutron porosity curve (or in a more quantitative way, the hydrogen index curve) and the density porosity overlay in liquid-filled formations, there is a large separation between the two curves in gas. In the presence of gas, the hydrogen index drops, indicating lower liquid-filled porosity. The density drops as well, which corresponds to larger liquid-filled porosity.
Gas detection and quantification can be based on a measurement of the bulk density (assuming that the matrix density is known) and of the (thermal or epithermal) neutron porosity or of the HI of the formation, i.e., at least two independent measurements are used to obtain gas saturation. Neutron porosity and HI are measurements of the hydrogen content of the formation and therefore related to the fluid in the pore space. The bulk density measurement can be based on the measurement of the electron density of the formation and its transform into bulk density, and is sensitive to the matrix density, the formation porosity and the density of the fluid in the pore space. The neutron measurement can be used to obtain the HI of the formation, while the density measurement contains information on the bulk density of the formation and, if the matrix and fluid density are known, of the porosity of the formation.
Attempts have been made at using traditional gamma-gamma density tools in casing. However, the results have been unsatisfactory and the size of the OH tools limits the measurement to large casing inner diameters. Attempts have been made to replace the OH measurement by a cased hole measurement using a pulsed neutron tool. Such a tool would provide HI and a second quantity related to formation density, but remain unaffected by HI. Various pulsed neutron tools use at least two gamma-ray detectors at different spacings from the pulsed neutron source. The detectors will detect gamma-rays resulting from inelastic interactions of high energy neutrons with the material surrounding the tool, gamma-rays from the capture of epithermal and thermal neutrons as well as gamma-rays from the activation of formation materials.
The ratio of capture count rates of two detectors can be used for a measurement of the neutron porosity and/or HI of the formation, in a fashion similar to the porosity measurement obtained by compensated neutron logging tools, based on the ratio of thermal or epithermal neutron count rates at different detector spacings from the neutron source. Inelastic gamma-rays result from the interaction of fast neutrons (>1 MeV) with material surrounding the tool. The number of inelastic gamma-rays created in the formation is a function of the formation composition and in particular a function of the density of nuclei (atom density) in the formation weighted by their neutron inelastic cross section. In addition, the transport of the gamma rays after their creation is also sensitive to the electron density of the formation. Since the fast neutron slowing down process is a function indicative of HI, the inelastic gamma-ray creation is also a function of HI.
Many pulsed neutron methods for the detection and quantification of gas have been proposed over the years. In proposed methods, detection and quantification is based on the measurement of inelastic gamma-rays, which are assumed to have sensitivity to the formation density, and therefore the presence of gas. The measurement of inelastic gamma-rays is made difficult by two factors.
First, the gamma-rays observed during the neutron burst are not solely from the interactions of fast neutrons, because there are both epithermal and thermal capture during the burst. To obtain a “net inelastic” count rate, the capture background may be subtracted. Such a subtraction can be difficult and inaccurate, and the problem is exacerbated if the neutron burst is not well defined and if the detectors and material surrounding the detectors exhibit noticeable epithermal capture cross sections.
Second, the assumption that the inelastic count rate, even after a perfect subtraction of the capture background, is providing a signal that is only related to the formation density and not its HI is not correct. As mentioned above, the production of inelastic gamma-rays is influenced not only by the formation density but also by the formation HI.